Coherent Passive Optical Networks for 100G/λ-and-Beyond Fiber Access: Recent Progress and Outlook

Coherent optics is considered a promising candidate for realizing single-wavelength passive optical networks (PONs) at 100 G/lambda and beyond. It has been a game changer for enabling ultra-high-speed data transmission in long-haul and metro networks. Interest in coherent optics is at an all-time high for application to short-reach access networks. The use of a local oscillator in coherent beating with received signals results in superior receiver sensitivity and thereby an extended power budget that supports high-capacity, long-reach, and large-split-ratio access. It has also been demonstrated to be beneficial for flexible deployment and adaptive access by advanced modulation formats and powerful digital signal processing. In this article, we review use cases, challenges, and recent progress in high-speed coherent PONs for 100 G/lambda, and beyond. We also provide technical insights into key enabling technologies in coherent PONs, including innovations for simplification and optimization of coherent optics and coherent upstream burst-mode detection. Finally, we discuss the outlook for coherent PONs toward realizing a rate-flexible and adaptive fiber access network. The architecture of a rate-flexible single-wavelength time-and-frequency-division multiplexing coherent PON based on digital subcarrier multiplexing technology is introduced. In addition, the adaptive fiber access network enabled by the adaptive modulation and coding scheme is also discussed.